Here is a list of free MMOGs/MMORPGs.
Runescape is a MMOG made by Jagex and is very popular. There has been over 130M accounts created since Runescape was launched. There is standard Runescape, HD Fullscreen Runescape, HD Resizable Runescape and HD Runescape.
World of Warcraft
World of Warcraft (WoW) is a game made by Blizzard Entertainment. It is extremely popular and has very good graphics.
AQ Worlds is a game made by Artix Entertainment and has only been up for a little while. It is still in its tesing stages but is still good.